The US EPA currently restricts the total nitrate plus nitrite concentration (as nitrogen) in drinking water to less than 10 mg/L, and has reported that exposure to perchlorate should not exceed the 4-18 xcexcg/L range in order to provide an adequate health protection margin. High nitrate/nitrite and perchlorate levels in drinking water have been linked to serious health problems and sometimes death. The concern over nitrate/nitrite has been driven by increasing levels of these contaminants being detected in drinking water supplies originating from both inorganic and biological sources. Inorganic sources include intense agricultural practices which contribute both ammonium and potassium nitrate fertilizers, explosives and blasting agents, heat transfer salts, glass and ceramics manufacture, matches, and fireworks. Biologically derived organonitrogen compounds are converted to nitrate in natural waters relatively rapidly. The main source for compounds is human sewage and livestock manure, which are not effectively removed by current treatment systems. Once in the environment, nitrates move rapidly into ground water reservoirs which supply drinking water. Recent studies have indicated the presence of perchlorate in drinking water wells throughout the Western United States. Perchlorate contamination of ground and surface waters originates from the manufacture and destruction of ammonium perchlorate; a strong oxidant used in the aerospace, munitions, and fireworks industries. Practical and efficient methods to treat water contaminated by these pollutants do not currently exist and are needed to insure a safe drinking water supply.
Ammonium perchlorate has been widely used by the aerospace, munitions, and fireworks industries, resulting in widespread soil and water contamination. The end of the Cold War has left the Department of Defense (DOD) with approximately 140 million pounds of ammonium perchlorate to be disposed of between 1993 and 2005. Perchlorate contamination in drinking water wells was first detected in early 1997 in northern California. These findings prompted further investigation, and perchlorate was detected in southern California wells, the Colorado River, Las Vegas wells, and Lake Mead. Although Federal drinking water standards do not currently exist for perchlorate, it has been placed on the current Drinking Water Contaminant Candidate List by the EPA.1 There is significant concern over the human health effects of perchlorate due to its known interference with the thyroid gland""s ability to utilize iodine and produce thyroid hormones. As a result of these findings, the California Department of Health Services (DHS) adopted an action level for perchlorate in drinking water of 18 xcexcg/L.2 This level was based upon an earlier recommendation by the EPA for a provisional reference dose (RfD) of 14 mg/kg/day.3,4 In August of 1997, DHS notified drinking water utilities of their intent to treat perchlorate as an unregulated contaminant that must be monitored and reported to the DHS.
Perchlorate contamination has been detected in eastern Sacramento County at Aeroj et General Corporation""s facility, a site previously owned by McDonnnell-Douglas, and a site previously owned by Purity Oil Company. Due to the presence of volatile organic chemicals (VOCs), contaminated groundwater at the Aerojet General site was treated and then reinjected into aquifers in the area. Monitoring of the reinjected water indicated that it contained up to 8000 xcexcg/L of perchlorate. In February 1997, perchlorate was detected in drinking water wells in the Rancho Cordova area at levels as high as 280 xcexcg/L. In July of 1997, DHS tested 62 wells in northern California, and detected perchlorate in 13. Of the 13, eight exceeded the 18 xcexcg/L action level. Also, groundwater monitoring wells associated with the United Technologies Corporation in Santa Clara County yielded perchlorate concentrations as high as 180,000 xcexcg/L, although no contamination of the drinking water systems was evident.
In southern California, perchlorate contamination has been detected in wells at Loma Linda and Redlands (5-216 xcexcg/L) associated with past operations of the Lockheed Propulsion Company. Perchlorate was also detected at low levels in wells at Riverside, Chino, Colton, Cucamonga, and Rialto. In Los Angeles County, perchlorate has been detected in concentrations ranging from 4 to 159 xcexcg/L in the areas of Azusa, Baldwin Park, Irwindale, La Canada, Flintridge, La Puente, Newhall, Pasadena, Santa Clarita, and West Covina. The perchlorate contamination was thought to originate from Aerojet (Azusa), the Azusa landfill, the Whittaker-Bernite site (Santa Clarita), and two Superfund sites, the Jet Propulsion Laboratory (Pasadena) and the Baldwin Park Operable Unit.
Outside of California, perchlorate has been found at levels of 5 to 9 xcexcg/L in the Colorado River. These findings prompted testing in Nevada. In August, Nevada sites were found to contain perchlorate levels of up to 13 xcexcg/L in drinking water wells, 1700 xcexcg/L in the Las Vegas Wash, and 165 xcexcg/L in Lake Mead. Monitoring wells in areas of ammonium perchlorate manufacturing were then found to have levels of 630,000 to 3,700,000 xcexcg/L. In Utah perchlorate was found at levels of 200 xcexcg/L at a rocket motor manufacturing facility.
Potassium perchlorate""s health effects were originally discovered due to its use in the 1950""s to treat Graves"" disease, an autoimmune disorder in which patients develop antibodies to the thyroid stimulating hormone (TSH) receptors in the thyroid resulting in hyperthyroidism. Perchlorate was found to displace iodine in the thyroid, causing a decrease in production of triiodothyronine (T3) and tetraiodothyronine (T4), two regulating hormones which control TSH production. This effect has been shown to be reversible, with the perchlorate eventually being expelled from the thyroid. A study by Stansbury and Wyngaarden5 was performed on Graves"" disease patients following a single dose of perchlorate. Studies by Godley and Stansbury, Crooks and Wayne, Morgans and Trotter, Hobson, Johnson and Moore, Fawcett and Clarke, Krevans et al., Gjemdal, and Barzilai and Sheinfeld followed perchlorate administration to Graves"" disease patients for periods up to several weeks.6-14 Only one case by Connell15 reported long term treatment in one patient for 22 years. Doses of perchlorate in these studies ranged from  less than 1 mg/kg/day5 to  greater than 20 mg/kg/day7 with the typical exposure being 6-14 mg/kg/day. The observable effects of perchlorate include blocking of iodine uptake and discharge by the thyroid,5 gastrointestinal irritation and skin rash,6,7 and hematological effects including agranulocytosis and lymphadenopathy.7,8 Seven cases of fatal aplastic anemia were reported at the same dose level, 6-14 mg/kg/day, at which other side effects occurred.9-14 
Following these early studies, the effects of perchlorate exposure on healthy volunteers were studied by Burgi et al.16 in five subjects for eight days at 9.7 mg/kg/day dosage levels. Brabant et al.17 studied five subjects dosed with 12 mg/kg/day of perchlorate for four weeks. Both of these studies observed effects on the thyroid at these levels. Studies in laboratory animals have included administration of perchlorate for four days by Mannisto et al.18 and for two years by Kessler and Krunkemper.19 The animal studies only examined thyroid effects at dosage levels too high to evaluate the perchlorate level defined as the no observable adverse effects level (NOAEL).
The initial effort to establish a RfD for perchlorate was undertaken by the Perchlorate Study Group (PSG), a consortium of companies that use and/or manufacture perchlorates. The PSG submitted a provisional perchlorate RfD) to the EPA""s National Center for Environmental Assessment Office (NCEAO) in 1995. The critical health effect cited in the PSG report was the interference with the thyroid functioning including the release of iodine from the thyroid, inhibition of iodine uptake by the thyroid, increased thyroid weight and volume, increased TSH levels, and decreased T3 and T4 thyroid regulating hormone levels. The PSG""s approach to a perchlorate RfD was to select a dose level that represented the highest level tested at which no adverse effects were observed. The critical study used by the PSG in their assessment was by Brabant.17 The PSG report recommended a RfD of 12 mg/kg/day.
In response to this report, the NCEAO derived the provisional RfD of 1xc3x9710xe2x88x924 mg/kg/day later used by the DHS in their recommendations. This RfD was based upon the NOAEL for potassium perchlorate in the functioning of the thyroid combined with uncertainty factors designed to account for sensitive populations, the less than chronic nature of the studies, and database deficiencies.2,3,5 The critical study used in this assessment was that of Stansbury and Wyngaarden which was the only study to report a NOAEL for humans.5 The NOAEL dosage was established at 0.14 mg/kg/day based on the release of iodine from the thyroid. To account for unknowns in the risk assessment process, the U.S. EPA used uncertainty factors (UF) to evaluate the NOAEL. An UF of 1000 was applied to perchlorate based upon 10 for a less than chronic study, 10 for database insufficiencies, and 10 for the protection of sensitive individuals. This resulted in a RfD of 0.00014 mg/kg/day, and a 4 xcexcg/L drinking water limit (70 kg average weight and two liters of drinking water per day). The U.S. EPA later reviewed the findings and available data, and changed the database uncertainty factor to 3, resulting in a higher RfD of 0.0005 mg/kg/day. Using the two UF analyses, the U.S. EPA concluded, xe2x80x9cuntil adequate chronic data becomes available that addresses the effects of perchlorate on the hematopoietic system (i.e., bone marrow), we feel that the provisional RfD is in the range of 1 to 5xc3x9710xe2x88x924 (i.e., 0.0001 to 0.0005) mg/kg/dayxe2x80x3 or 4-18 xcexcg/L of perchlorate in drinking water.
After review of earlier attempts to establish a RfD, the International Toxicity Estimates for Risk (ITER) Peer Review Panel concluded in March of 1997 that the database for perchlorate exposure was inadequate for the development of a RfD and that additional studies were required to establish a RfD. Due to this recommendation, the PSG and the U.S. Air Force obtained funding to support new studies to assess the toxicities of perchlorate.20 In September of 1997, the first study was initiated. New laboratory animal studies are in progress or about to begin at this time to determine the effects of perchlorate ingestion on neurobehavioral development, receptor kinetics, developmental fetal skeletal abnormalities, ADME (absorption, distribution, metabolism, and elimination), mutagenicity/genotoxicity, reproductive, and immunotoxicity. These studies will be used to fill in holes in the perchlorate database that have made it difficult for the EPA to set a RfD. The findings of these studies will be used to establish a new RfD.
Nitrate and nitrite levels in drinking water have also received intense regulatory scrutiny in the past due to their potential to cause serious health problems especially in infants and the elderly. High nitrate/nitrite levels in drinking water have been linked to serious illness and sometimes death. In infants, the conversion of nitrate to nitrite by the body can interfere with the ability of blood to carry oxygen. Under exposure to excessive nitrate levels, an acute condition can occur leading to shortness of breath and blueness of the skin (i.e., xe2x80x9cblue baby syndromexe2x80x9d or methemoglobinemia). In its acute form, this may lead to rapid health deterioration over a period of days. Chronic exposure to high levels of nitrate/nitrite can lead to diuresis, and increased starchy deposits and hemorrhaging of the spleen. Current EPA regulatory limits include a Maximum Contaminant Limit (MCL) of 10 mg/L (as nitrogen) and a 10-day Health Advisory Limit (HAL) of 10 mg/L.21 For example, a safe short term exposure for a 10 kg child consuming 1 liter of water per day over a ten day period would be 10 mg/L of total nitrate plus nitrite.
Part of the concern over nitrate/nitrite has arisen from the increasing levels of these contaminants detected in drinking water supplies. Since most nitrogenous chemicals in natural waters are converted to nitrate, all sources of combined nitrogen especially organic nitrogen and ammonia should be considered as nitrate sources. Due to its high solubility and weak retention by soils, nitrates are very mobile and move into groundwater reservoirs (i.e., aquifers) at a rate comparable to that of surface water. Biological degradation of nitrate by anaerobic denitrification reactions to form elemental nitrogen and ammonia is slow so that nitrate persists in the environment. In particular, organic nitrates originating from human sewage and livestock manure are potential sources for this type of ground water contamination. In the latter case, feedlots represent a large point source of this type of organic nitrogen pollution. Intense agricultural practices also contribute through the use of ammonium and potassium nitrate as fertilizers. Other inorganic sources of nitrate contamination include explosives and blasting agents, heat transfer salts, glass and ceramics manufacture, matches, and fireworks. The quantity of nitrate/nitrite released into the environment between 1987 and 1993 in the top fifteen states totaled 2.68xc3x97107 kg for water releases and 2.42xc3x97107 kg for land releases. Of these, nitrogeneous fertilizer contributed xcx9c44% of the total while industrial sources accounted for xcx9c30%.
The widespread occurrence of perchlorates and nitrate/nitrites in ground and surface water combined with the concern expressed by California""s DHS and the U.S. EPA clearly demonstrate the need for new more efficient technologies to eliminate these inorganic contaminants from drinking water. Numerous technologies have been investigated for the destruction of perchlorate, but none of these provide an economical process for treating drinking water to reduce these contaminants to levels below regulatory limits. Ion exchange, reverse osmosis, and electrodialysis are current methods used to remove nitrate/nitrite from drinking water, however, in all cases nitrate/nitrite are concentrated into a brine which then must be disposed of in an appropriate repository. Assurance of safe drinking water supplies in the future will require the development of technology which can eliminate both of these inorganic contaminants from drinking water without producing additional wastes.
The decomposition of perchlorate by biological, physicochemical, electrochemical, and thermal processes has been the subject of numerous patents.26-35 Included in these are a microbiological treatment using Vibrio dechloraticans Cuznesove B-1168 being fed acetate, ethanol, glucose, and other sugars in the absence of oxygen. The Air Force has investigated the destruction of ammonium perchlorate using Wolinella succinogenes HAP-1, an anaerobic microbe.20 Thermal destruction of highly concentrated perchloric acid solutions and perchloric acid in the vapor phase is also well known.26-30 The thermal decomposition of perchloric acid has been measured at moderate temperatures from 295-322xc2x0 C.30 Physical processing has been employed in which evaporation and precipitation of KClO4 were used to remove perchlorate.31,32 Electrochemical methods have also been used to reduce perchlorates to lower oxidation state chlorine compounds.33,34 Electrochemical reduction of perchloric acid solutions has been demonstrated using a titanium cathode.35 Prior to the current research, the catalytic reduction of perchlorates has not been actively pursued.
The elimination of nitrate and nitrite from water has been widely studied.36-40 The primary and traditional treatment methods have been based on nitrification and denitrification using different groups of bacteria under aerobic and anaerobic conditions.36-38 Chemical and physical processes such as reverse osmosis, ion exchange, and electrodialysis have all been considered as physico-chemical means to remove nitrate/nitrite from water.39,40 The use of an aqueous phase catalyst in combination with an organic reductant has not been considered.
Many aqueous phase catalytic oxidation studies have been performed using dissolved molecular oxygen as the oxidant and a variety of organic contaminants as reductants. The primary objective in these studies has been the destruction of aqueous phase organic contaminants. In effect, these studies mirror the proposed perchlorate destruction process with the exception that the contaminant that is being destroyed has changed from the reductant to the oxidant.41-64 
A process is provided for destroying contaminants in a contaminant containing aqueous stream. In the subject process, the contaminant-containing aqueous feed stream preferably comprises a contaminant-containing aqueous feed stream or aqueous brine feed stream.
The process of the present invention comprises providing the contaminant-containing aqueous feed stream including an initial amount of at least one of a group of contaminants including perchlorates, nitrates, and nitrites. In a preferred process of this invention for the destruction of perchlorate contaminants, an oxidation-reduction process is employed in which it is believed that the oxidation state of chlorine in the perchlorate (+7) contaminant is lowered, forming predominantly chloride (xe2x88x921). In a preferred process for the destruction of nitrate and nitrite contaminants, it is believed that the oxidation state of nitrogen in the nitrate (+5) and nitrite (+3) contaminants is lowered, forming elemental nitrogen (0).
A reducing agent is provided in the contaminant-containing aqueous feed stream. The reducing agent can be present therein in sufficient amount to facilitate the catalytic oxidation-reduction of the present invention. However, the subject process typically includes adding a non-toxic reducing agent to the contaminant-containing aqueous feed stream. The preferred reducing agents are organic reducing agents, more preferably low molecular weight polar organic species which are highly soluble and have a terminal carbonxe2x80x94oxygen bond. Most preferably, the reducing agent can comprises any one of a carbohydrate, an alcohol or an organic acid, more preferably ethanol or acetic acid.
There are also preferred inorganic reductants including dissolved hydrogen and ammoniacal nitrogen species (i.e., NH3 and NH4+). Other inorganic species such as hydrogen peroxide, urea, chloramines, or hydrazine hydrochloride which form oxidized by-products that are soluble may also be utilized as reducing agents.
When organic reducing agents are utilized, carbon dioxide and water are the predominant by-products from oxidation of the reductant. If inorganic reducing agents are utilized, then the chief by-products formed depend on the reducing conditions and the specific reducing agent. For example, the oxidation of hydrogen forms hydronium ions, while the oxidation of ammonia forms water, hydronium ions, and nitrogen.
Next, the reducing agent-containing, contaminant-containing aqueous stream is subjected to a heating step. The temperature to which the reducing agent-containing, contaminant-containing aqueous stream is typically raised is to a temperature of not more than about 250 degrees C., preferably to a temperature of not more than about 200 degrees C., more preferably to a temperature of not more than about 150 degrees C., and most preferably to a temperature of not more than about 50 degrees C.
The heated contaminant-containing aqueous stream is then contacted with an oxidation-reduction catalyst for a period of time sufficient for reducing the initial amount of any of the perchlorates, nitrates, and nitrites contaminants. Preferably, the step of contacting the reducing agent-containing, contaminant-containing aqueous stream with the oxidation-reduction catalyst is typically conducted for a period of time of not more than about 500 seconds, preferably not more than about 300 seconds, more preferably not more than about 150 seconds, and most preferably not more than about 50 seconds.
The oxidation-reduction catalyst is preferably a metallic oxidation-reduction heterogeneous catalyst. Oxidation-reduction catalysts can comprise chemically robust, high surface area supports impregnated with a metal, metal oxide, or with mixtures of metal salts which are subsequently reduced to metallic form. The supports are stable in aqueous solutions at the above-described reduction temperatures. The preferred supports are zirconium dioxide extrudates.
The heated contaminant-containing aqueous stream can be subject to pressure, as well as temperature, when it is contacted with an oxidation-reduction catalyst. The preferred pressures employed during the oxidation-reduction sequence is typically up to about 40 atmospheres, preferably up to about 10 atmospheres, more preferably up to about 3 atmospheres, and most preferably up to about 1 atmospheres.
The supports generally have a surface area of at least 20 m2/g, preferably at least 25 m2/g, more preferably at least 30 m2/g, and most preferably at least 35 m2/g. Moreover, the particle size of the support material is preferably up to about 2 mm, more preferably up to about 3 mm, and most preferably up to about 4 mm.
These oxidation-reduction catalysts of the present invention exhibit high activity towards the oxidation of dissolved organic species and towards the reduction of molecular oxygen and other suitable oxidants such as perchlorate, nitrate, and nitrite materials. The preferred metallic materials employed in the oxidation-reduction catalyst of this invention are platinum, palladium, and ruthenium.
The subject oxidation-reduction catalysts suitable for the process of this invention are high activity catalysts provided that the support material remains stable at the reaction conditions. Typical oxidation-reduction catalyst of this invention can comprise platinum and/or palladium and/or ruthenium catalytic metals supported on other materials. Such support materials can include titanium dioxide, cerium oxide, aluminum oxide, silicon dioxide, silicon carbide, and activated carbon. The platinum loading on typical supports employed in the process of this invention may be as high as 20% by weight, based on the weight of the support material, although a cost-benefit analysis generally can reduce this value by a factor of ten for commercial application. The ruthenium loading on these supports is preferably up to about 5.0% by weight. The palladium loading on these supports is less than about 2.0% by weight, based on the total weight of the support material.
Oxidation-reduction catalysts suitable for this process can also include other metals, metal oxides, or mixed metal oxides supported on a variety of materials. Typical metals in this group include copper, iron, cobalt, and nickel. Typical metal oxides include copper oxide with manganese oxide, chromium oxide with iron oxide, and chromium oxide with cobalt oxide. Metal and metal oxide loadings are up to about 4.0% by weight, based on the weight, based on the weight of the support material.
A preferred oxidation-reduction catalyst for the destruction of perchlorate, nitrate, and nitrite using organic reductants comprises platinum and ruthenium on zirconium dioxide. The optimal platinum loading based on performance-cost evaluation is between about 0.5 and 2.5% by weight. The optimal ruthenium loading based on performance-cost evaluation is between about 0.1 and 0.5% by weight, based on the weight of the support.
A preferred oxidation-reduction catalyst for the reduction of perchlorate, nitrate, and nitrite using dissolved hydrogen as the reductant comprises platinum, palladium, and ruthenium on zirconium dioxide. When utilizing hydrogen as the reductant, palladium containing catalysts exhibit higher reaction rates than non-palladium containing catalysts due to the high hydrogen solubility in palladium, combined with the consequent availability of hydrogen at the palladium surface. The optimal loading based on performance-cost evaluation for platinum is between 0.5 and 2.5% by weight, for palladium is between 0.5 and 2.0% by weight, and for ruthenium is between 0.1 and 0.5% by weight of the support.
A variety of reactor configurations may be utilized to perform the aqueous phase catalytic oxidation-reduction process of this invention. The preferred catalytic reactor comprises a plug flow reactor containing the catalyst bed. Reducing agents are added at the inlet at ambient temperature and pressure. The aqueous stream is then pressurized, heated, pumped through the reactor, and cooled at the outlet. Reactor temperature control and overall energy efficiency is improved by coupling inlet and outlet flows through a regenerative heat exchanger. Depending on the contaminated stream and reducing agent, perchlorate, nitrate, and nitrite will be reduced in the presence of a stoichiometric excess of the reducing agent at a kinetically determined residence time within the catalyst bed. When hydrogen is the reducing agent of choice, a stoichiometric excess of gaseous hydrogen is injected into the pressurized stream at a concentration sufficient to reduce the contaminant species at the contact time provided. The reactor operating pressure is adjusted to maintain single phase operation and to supply sufficient dissolved gas to maintain a stoichiometric excess reductant. Operating pressures and temperatures are maintained in a range as previously described above.
A preferred oxidation-reduction catalyst for the reduction of perchlorate, nitrate, and nitrite using ammonium cations as the reductant comprises platinum and ruthenium on zirconium dioxide. The optimal loading based on performance-cost evaluation for platinum is between 0.5 and 2.5% by weight, and for ruthenium is between 0.1 and 0.5% by weight of the support.
An aqueous phase catalytic reduction (APCAR) process will catalytically reduce perchlorate, nitrate, and nitrite using a variety of organic and inorganic reductants at low temperatures between 25 and 125xc2x0 C. in water. An APCAR process will catalytically reduce perchlorate, nitrate, and nitrite using a variety of organic and inorganic reductants at temperatures above 150xc2x0 C. in brines which contain between 1 and 12% by weight sodium chloride.
When organic reductants are utilized, carbon dioxide and water are the predominant reaction by-products. If inorganic reductants are utilized, then the chief by-products formed depend on the reduction conditions and the specific reductant.
When the heated contaminant-containing aqueous stream is contacted with an oxidation-reduction catalyst for a period of time sufficient, the initial amount of any of said perchlorates, nitrates, and nitrites contaminants is substantially reduced. More specifically, the extent of the above-described substantial reduction is preferably by at least about 90%, more preferably by at least about 92%, and most preferably by at least about 95%.